A 53 year-old male visited our hospital for evaluation of his leukocytosis, which was first diagnosed more than 6 years previously. He was asymptomatic and there were no remarkable findings on physical and laboratory examinations except for the lymphocytosis. Abnormal lymphocytes with deep folded nuclei were seen on light microscopy, whose phenotype was CD3+, CD4-, CD8-, CD7-, CD16 , CD56-, CD45RO+ and CD45RA- . Electron microscopy revealed 'cerebriform nuclei' which were characteristic of Sézary cells. Adult T cell leukemia (ATL) and Sézary syndrome (SS) were ruled out because of the negative HTLV-1 test and the absence of skin lesions, respectively. T-prolymphocytic leukemia (T-PLL), which is characterized by a marked increase in leukocytes having a CD7-phenotype and a progressive fatal course, was also excluded. Recently, the TCL1 onco-protein has been shown to be overexpressed in progressive T-PLL but not in other mature T cell leukemias including Sézary syndrome. Peripheral mononuclear cells in the present patient did not overexpress TCL1. In its morphology and phenotypes, our case resembled 'Sézary cell leukemia (SCL)' but the clinical course was much more indolent. This case did not match any of the mature T cell leukemias defined in the WHO classification.